The invention relates to a method for focusing a reproducing objective, for instance for distance measurements, wherein multiplication of the image of the entrance pupil or iris is achieved by means of image splitting optical elements located in the plane of the object image.
The state of the art of follow up systems for focusing the slide in a slide projector may be ascertained by reference to U.S. Pat. Nos. 3,399,595 and 3,517,992, and the follow up systems are useful in focusing the objective of the present invention. The disclosures of these patents are incorporated herein.
With regard to photographic cameras, it is known to mount pyramidal grids or rasters on the pane of the viewer for the purpose of focusing the objective or for visual assessment of the same. For each direction of surface inclination of the trilateral or quadrilateral pyramid, there will be a laterally shifted reproduction of the objective pupil, provided a lens reproducing the object pupil is mounted in the vicinity of the grid plane. The observing (ocular) puil covers part of the domain of the overlapping pupil images. However, since the ocular pupil is of a different relative position in each of the pupil images, many object images shifted by parallax and corresponding in number to that of the pupil images will occur when the focusing plane is moved out of the screen plane. Errors in camera objective setting therefore are noted by the fuzzy, fringy nature of the transitions in image contrasts.
Under such conditions, it becomes difficult to subjectively assess focus, especially if there is great depth of field of the reproduction through the camera objective.